The present invention is related to laser beam focussing devices and more particularly to an apparatus adapted for focussing and time averaging the intensity distribution of a beam of radiation on a workpiece at high frequency.
Material processing applications typically require rapid, controlled scanning of a focussed beam of high power radiation over a surface of a workpiece. The rapid, controlled scanning of the radiation is required to reduce the average intensity of the beam energy input onto the workpiece while maintaining a high instantaneous intensity of radiation at the interaction zone to promote effective coupling of the radiation with the workpiece. In a typical example utilizing high power radiation from a carbon dioxide laser for welding aluminum, an incident power density of approximately 5.times.10.sup.6 watts per square centimeter is required to overcome the initially high surface reflectivity of the aluminum material and to establish a deep penetration welding condition therein. Once the deep penetration welding condition has been established, however, the efficiency of energy coupling increases dramatically. The increased energy absorption together with the relatively modest energy requirements for fusing aluminum (due to low density and melting point) lead to substantial overheating of the weld zone. In addition to causing sporadic vaporization and material expulsion, the high molten material temperature promotes hydrogen solubility and resultant weld porosity.
Davis et al. in U.S. patent application Ser. No. 209,940 filed Nov. 2, 1980, a continuation of Ser. No. 1,038, now abandoned, and held with the present application by a common assignee, discloses a mechanical rotating apparatus adapted for rotating a beam of radiation about its propagation axis to effectively time average the azimuthal intensity distribution of the radiation incident on a workpiece. The beam undergoes multiple reflections within the apparatus such that the beam rotates at twice the rotation frequency of the apparatus. Displacing the beam exiting the rotating apparatus from the rotation axis effectively time averages the azimuthal and radial intensity distribution. This unit, however, does not adapt to moving the focussed spot and is limited to the frequencies reasonably attainable by mechanical rotation.
Another method of time averaging the intensity distribution of a high power beam of radiation is to oscillate the beam across the beam-radiation interaction zone. This method has been clearly demonstrated in electron beam technology wherein electron beams due to their electric charge can be readily scanned at high frequency over an interaction zone with electric or magnetic means. Electrooptical and mechanical scanning means are available for low frequency oscillation of a beam of laser radiation having low power. However, low frequency oscillation of the beam to obtain time averaging of the intensity distribution is inadequate for most laser welding applications since the interaction time of the material is more rapid than the time required to scan the beam across a weld zone. Under such conditions a narrow, spiral weld bead will be formed rather than the desirable broadened, linear bead.
To obtain effective time averaging of the intensity distribution for welding purposes, the beam must be oscillated across the weld zone in a time short compared to the characteristic thermal conduction time of molten metal such that the reaction of the material with the radiation is characterized by the interaction of a beam having an intensity averaged distribution with the material. For aluminum material this typically requires oscillating the beam at a frequency in excess of one thousand hertz.
Prior art techniques of oscillating a beam of high power radiation at high frequencies are not suitable. Laser mirrors adapted for focussing high power radiation typically have substantial bulk and/or require cooling. Direct mechanical oscillation of these optical elements at frequencies of one thousand hertz or greater requires prohibitive driving forces. The present invention discloses an apparatus obviating this difficulty.